The present invention relates to a drill element for percussion drilling, e.g., a drill rod, a coupling sleeve or a drill bit, said drill element having a female or a male cylindrical screw thread having an external diameter less than 30 mm, preferably less than 25 mm, and having one single entry. The drill element is intended to be coupled up to another element included in a percussion drill string.
The invention primarily refers to drill elements having threads of the type called round or rope threads. These threads have a constant radius of curvature at both the top and the bottom of the thread. Portions of the thread have a thread surface almost parallel to the longitudinal axis of the drill element. In order to prevent those surfaces from being wedged-up, the radius of curvature at the top of the male thread and the bottom of the female thread is made somewhat larger than the radius of curvature at the bottom of the male thread and the top of the female thread, respectively. However, the invention is generally applicable on drill elements having cylindrical screw threads having one single entry.
The rope threads according to the prior art have turned out to perform well in connection with relatively large thread diameters when used together with rock drills having low to medium high drilling power. The thread profile in such cases is kept unchanged independent of the thread diameter. However, it has turned out that for smaller thread diameters, primarily the standard diameters of 22 mm and 25 mm, the life span and the unscrewing characteristics are not satisfactory. These negative characteristics become even more acute if the drilling equipment is used at a low working pressure, i.e., a working pressure less than the normal one of 7 bar. The main reason for the rapid wearing out of the thread semms to be that the thread is too easy to unscrew and thus is shaken loose at regular intervals during drilling. This causes the creation of so-called pittings on the shoulders.
The object of the present invention is to provide a drill element that for smaller thread diameters has a longer life span and improved unscrewing characteristics compared to rope threads of standard type.
Another object of the invention is to provide a drill element that has a long life span and good unscrewing characteristics also at a low working pressure